High-frequency oscillator circuit for induction heating apparatus



Nov. 30, 1948. V w SHERMAN A 2,454,845

El' L HIGH-FREQUENCY OSCILLATOR CIRCUIT FOR1 INDUCTION HEATING APPARATUSFiled May 4, 1945 Hes 5R26 'Tll-*I es "o ,Belg- Impl- 47mm l' PatentedNov. 30, 1948 HIGH-FREQUENCY OSCILLATOR CIRCUIT FOR INDUCTION HEATINGAPPARATUS Vernon W. Sherman, Summit, and Carson Mc- Keag Wheeler,Montclair, N. J., assignors to Federal Telephone & Radio Corporation,New- York, N. Y., a corporation of Delaware Application May 4, 1943,Serial No. 485,582

(Cl. Z50-36) 6 Claims.

This invention relates to high frequency oscillator circuitsparticularly adapted for the energzing of induction heating apparatussuch as is employed for example in the induction heating or hardening ofsteel or in like use wherein substantially the maximum load is employedas the normal operating condition. In induction heating of this type thework piece to be hardened or otherwise treated is placed within theheating coil prior to starting of the oscillator which, coupled with thef act that a peak load is presented by the work during the ini-tialperiod in the heating operation, establishes a load condition whichmakes it ydifficult to initiate high frequency oscillation in theoscillator circuit. In the `desired economical operation under maximumload the condition may exist that an oscillator capable of sustainingoscillation With the applied load is normally unable to initiateoscillation under the same load level. The object of the presentinvention is to provide an improved Icir-cuit arrangement which willproduce a starting impulse or shock in a -grid circuit so as to star-tos-cillation in a heavily loaded tank circuit. The improved circuit isaccordingly designed to establish oscillation under the aforesaidlconditions of maximum load by the momentary appli-cation of a highvalue positive voltage pulse to the grid.

For the described purpose there is provided 'a supplemental energizingor shocking circuit including a positive voltage grid bias supply orsource which is controlled by a relay to be effective with the initialenergizing of the heating circuit and which operates to apply a positivepotential to the grid of the oscillation tube as a supplemental positiveimpulse to effect an abrupt change of plate current to be instrumentalin establishing or initiating the oscillation.

The improved circuit arrangement of the presen-t invention Will be morereadily, understood by reference to the accompanying drawings whereinthere is diagrammatically shown a conventional oscillator circuitarrangement with which is incorporated the relay controlled supplementaland positive voltage grid bias supply circuit for applying themomen-tary shocking voltage to initiate the oscillation.`

The yconventional induction heating -circuit arrangement as shown, ofthe Hartley oscillator circuit type, includes the oscillator tube Ahaving the plate p and grid g -connected to the oscillator circuit Oincluding connections 2 and 3 connected to the plate and gridrespectively, leads 4 and 5 provided with condensers 6 and 'I andconnected to parallel leads l8 and 9 of tank circuit T having includedtherein a primary energizing coil 1 Iand condenser I0 respectively. Thecoil r has the adjustable ground connection II to establish regenerationin the known manner. The condensers 6 and 'I provi-de a low impedancepath for the radio frequency current while forcing .the direct currentto iiow through the tube. The usual cathode c of the -oscillator tube isconnected to ground as shown. The input for the oscillator circuit is bythe lead I2 from a high ltension source having the usual radio frequencychoke I3 to prevent radio frequency current flowing into the hightension supply and a protective condenser I4 in a shunting connection isgrounded as shown. The load circuit is shown at L including the usualsecondary or coupling coil s. The normal negative grid b-ias voltage isdeveloped across resistor I9, and is applied thru connection I5 andthence thru radio frequency choke coil I8 to the grid g. Connection 2l]:to the lead I6 functions normally to provide a xed negative grid biasvoltage suicient to stop oscillation. This connects t-o the negativegrid potential source 22 by means of a relay switch R normally engagingthe -contact 25.

In accordance with the present invention there is further provided asupplemental circuit arrangement operative momentarily to impressapositive grid bias upon the oscillator tube resulting in an abruptchange of plate current in a manner and of a magnitude to be effectiveto start oscillation. For this purpose the relay R is of double throwtype adapted on opening of contact 25 to close a second contact 26 of Vapositive grid potential source 28 including batteries connected lto lead29 through resistance 30 and provided with l a shunted and groundedcondenser 3|. The relay switch R is operated by the closing of circuitconnections 32-33 having in series therewith the relay coil 35 andasuitable timer indi-cated at 36; this circuit being closed or initiatedby a push button or manual switch at a convenient loca-tion.

The improved circuit is accordingly adapted to establish oscillationunder the -condition of substantially maximum load by the momentaryapplication of an increased positive grid bias occurring upon thedischarge of condenser 3I. The -positive grid bias established by thedischarge of condenser 3| produces an abrupt rate of change of the platecurrent and since this rate of change -controls the magnitude of voltageacross the :tank -circuit it is the means for inducing a suflicientlyhigh voltage at the grid of the tube to initiate yand sustainoscillation. This shock voltage will rbe impressed `on the grid circuitimmediately upon the closing of the relay on contact 25 and thereafter,the duration of the sustained oscillation will be determined by thetimer 36 in accordance wi-th .the desired time of induction heattreatment of the Work piece or load as Will be readily understood.

The provisionnof condenser 3| 'insures that there is an-instantaneouspotential of sufficient f magnitude to produce a shock and startoscillations in the circuit. However, the condenser isla immediatelypartially dischargedi and thus them potential of grid g is reduced to 'anormal?valuecg.A

suilicient to sustain oscillations. These oscillations continue untilrelay switch n1:?. yis operated-sv :to -connect the negativepotentiallofcontact25jf`2 to .the grid, at which time theoscillationsfare-sup-Vrpressed.

What is claimed is:

1. In combination: an oscillator tube havingr anode, cathode and controlgrid electrodes; Va feedback oscillation circuit connected to saidelectrodes; a. source of voltage normallyconnectedto saidgrid, theypotential of .said .source havinga value which prevents oscillationbeing.V developed vi in said. oscillation. circuit; a supplementalsource of .voltagecomprising a capacitor, a resistance,.andascapacitorcharging means connectedin se. ries,v relation,thepotentialof said supplemental.. sourcewhavingfa value suicient to.initiate oscilf.;l lationscin said circuit.;.and means for .connectirgsaidi supplemental voltagesource to said` gridfor apredeterminedperiodof time.

to saidgrid to start oscillations in said circuit. and

means lfor disconnecting-,said .supplemental volte 1 age source'to stop4oscillations in said circuit;

3.1-A circuit foristarting;oscillations in a selffsi. excitedoscillatoncircuit including gan electronic tube` having ananode, cathodeand` control grid,... comprising,.in combinationz. a source of potentialof -such-fvalueiiasto prevent oscillation in said.. oscillator whenvsaidsource is connected between saidbcathodeand said control grid; asecondo? source. of-potential of such value 'as to initiate oscillationsf in y said oscillator circuitiwhenl con;

nectedbetween said cathode and.v said control grid; switching means fordisconnecting said first sourceof potential from `said' control.r grid.andi-...1 connectingsaid second source thereto; a resistor.connectedlbetweenvsaid second `source .of `potene tialand .saidswitching means; anda capacitor connected fromthe end of -said resistoradjacent. .I saidswitching means to said cathodejwherebys r, theoperation *of-said switching means *ffromsaidid c 2,026`,8.7.4"fw;Eitel. v n Jane', 19363 rst- Ato said second positionapplies high-poentential` to said controlgrid to vinitiate oscillationvsz'` andthereafter`said-voltage reduces. to a lower@4 value suiiici'ent toVsustainoscillation.v

4 anode, cathode and control grid electrodes; a feedback oscillationcircuit connected to said electrodes; a voltage source including adirect current source and a condenser charged thereby, the potential ofsaid voltage source having a value-sucientgtoinitiate oscillations insaid circuit; means for connecting saiddirect current voltage andcondenser in parallel to said grid to start oscillations in saidcircuit; said `connectingmeans .being such as thereby to discharge ,saidcondenser and to reduce the potential oi said source to a potential lessthan its initializvalue .butsuiiicient to permit sustained oscillationsin said circuit.

l 5. A circuit for starting oscillations in a selfexcited oscillatorcircuit including an electronic tube having-gananode, cathode andcontrol grid, comprising, in combination: a source of potential of -suchvalue as toprevent oscillation in said oscillator when said source isconnected between..said. cathode and said control grid;l a secondasourceof potential of suchvalueas -to ini. A tiateoscillations in saidoscillatorircuitwhen. connectedbetween saidc'athode and: said controlVgrids.;:switching means for disconnecting said..lv

rstf.: sourceyfromi said control. l grid and. iconv. nec-ting:saidssiecond. source ithereto; and. circuit, means connected to said.second source of ,po-

tential. operative upon commencement lof oscill' 4lations,...in:said..;oscillat'or circuit: to reducerthe4 potential; of `saidisecond4source to 5a potential lesslithany its z initial'` Value. 1 but`sufficient .to sustain. oscillations 1in said .oscillator circuit, whereby-operationof Isaid: switching means/from said i. fiirseto;said;secondi position applies a high.y pory tential` tol said,'controlgrid to initiate oscillation fr. r

and-thenreducesgsaidvoltage to a lowerfvalue duri-ngi thendesircdjperiodof joscillationl.

6. A circuit .for .starting oscillations in .fa Seli-fexcitedloscillator circuitxincluding an electronictubeghatzingan;-anodepcathode and control grid,V i, comprisinganCombination: `a source of potential 5,.; 0f .-Sllch-'iualue as atoinitiate oscillationsin said voscillatprvcircuitg when. connectedbetween .said ycatlnide..-1rr,1d; saidcontroL-g-rid; switchingmeansfor.Y connecting said sourceof .potential vto4 lsaid: .f control grid; aresistor connected;betweemsaid;.1 source of potential-and fsaid;Silvitchingil.means; ;-A and a; capacitor conneetedffrom-thetend .ofsaidf resistor. adjacent :asaiol-y switchingfmeans to said@cathodeqfwhereby Aoperationfof said switching meansifappliesma high;potential gto said control; grid-tto gin-(itiate oscillation 'andvthereafter said. voltagegis reduced-to a lower fvalue .suiicienttofsustain; oscillation.. Y

appearances v cI-'ren- Ovb Tlieciollowing references are of record-1inthe Non'lber...E Name Datei:`

2,118,414 Moullin:; May;24,z1938f 2,311,631 Bach Feb123zzt943p2,318,0611-wf Dailey i May-4, 1943 2,329,525: i Mittelmanfni JulyiZO,'1943 4. In fcombinationcfan oscillator:tubehaving 7o .fc2,330,"82i'1yHeppi; Sept. 28,'194311l

